WASHINGTON — The Pentagon has told the Obama administration that any military effort to seize Syria’s stockpiles of chemical weapons would require upward of 75,000 troops, amid increasing concern that the militant group Hezbollah has set up small training camps close to some of the chemical weapons depots, according to senior American officials.

The estimated size of the potential effort, provided to the White House by the military’s Central Command and Joint Staff, called into question whether the United States would have the resources to act quickly if it detected the movement of chemical weapons and forced President Obama, as he said in August, to “change my calculus” about inserting American forces into Syria. So far Mr. Obama has avoided direct intervention into the most brutal civil conflict to emerge from the Arab Spring uprisings, and the Pentagon assessment was seen as likely to reinforce that reluctance.

The White House on Thursday declined to comment on the Defense Department’s assessment.

The Pentagon has not yet been directed to draft detailed plans of how it could carry out such a mission, according to military officials. There are also contingency plans, officials say, for securing a more limited number of the Syrian chemical weapons depots, requiring fewer troops.

The discovery that Hezbollah has set up camps close to some of the depots, however, has renewed concern that as the chaos in Syria deepens, the country’s huge chemical weapons stockpiles could fall into the wrong hands. Hezbollah fighters have been training at “a limited number of these sites,” said one senior American official who has been briefed on the intelligence reports and spoke on the condition of anonymity. “But the fear these weapons could fall into the wrong hands is our greatest concern.”

So far, there is no evidence that Hezbollah, which is based in Lebanon but has become increasingly active in protecting the government of President Bashar al-Assad, is making any effort to gain control over the chemical weapons. Its decision to train fighters close to the major chemical sites, some officials speculate, could be rooted in a bet that their camps will not be bombed if the West believes there is a risk of hitting the stockpiles.

Mr. Assad has openly threatened to retaliate beyond his country’s borders if outside forces try to break the current stalemate to unseat him, and there is renewed concern about whether he or his proxies might use the chemical weapons as their last shield. Officials say that attacks along the borders with Turkey and Israel have forced the administration to consider the risks of Syria’s troubles spreading in the region.

Mr. Obama has been clear for more than a year that he would resist direct American intervention, but in August he said one circumstance would cause him to revisit that position. “A red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized,” he said at a news conference. “That would change my calculus.”

Mr. Obama brought those concerns up again in a news conference on Wednesday, saying that the United States was in close contact with Turkey and Jordan “and obviously Israel, which is having already grave concerns as we do about, for example, movements of chemical weapons that might occur in such a chaotic atmosphere and that could have an impact not just within Syria but on the region as a whole.”

The American concerns have been heightened by another sign that Mr. Assad may be arming himself to strike out — Syria’s continued imports of missile technology, even at a time when the Assad government is reeling under sanctions.

Syria already has a vast arsenal of missiles able to reach Turkey or strike Israel, and in the past it has provided Hezbollah with missiles. But American officials voice concern over even modest improvements in Syria’s missile stockpiles.

American intelligence and security officials, in interviews in recent days, said that the United States had picked up evidence that North Korea had resumed providing Syria with some missile technology, including assistance with Scud missiles.

A shipment of graphite cylinders that could be used in missiles and are suspected to have come from North Korea were found in May aboard a Chinese ship en route to Syria, Reuters reported Wednesday. North Korean technicians and engineers stationed in Syria have recently increased their efforts on a joint program to improve the Scud D missile’s accuracy and the warhead’s ability to defeat interceptors, IHS Jane’s International Defense Review reported in June.

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Given the chaos in Syria, and Mr. Assad’s daily effort to survive, it is not clear what condition the Syrian missile program is in. The Turkish military has expressed worry about Syria’s ballistic missiles and its chemical weapons stocks. Concerned about this potential threat, Turkey and NATO nations have informally been discussing the possibility that some of the alliance’s Patriot antimissile system could be sent to Turkey, which has no Patriot batteries of its own.

Independent analysts expressed concern that if Mr. Assad is backed into a corner, he could use or threaten to use missiles tipped with chemical weapons against the rebels, despite the threat of Western intervention if he did.

“There is credible information that the Assad regime has been upgrading and expanding its chemical weapons arsenal, which needs to be maintained,” said Emile Hokayem, a Middle East analyst at the International Institute for Strategic Studies. “A credible delivery capability is also needed, hence the North Korean angle.”

The estimation that it would take 75,000 troops to neutralize the chemical weapons grew out of what Mr. Obama, in his August news conference, referred to as extensive contingency planning for how the United States would respond if the chemical weapons were on the move or appeared vulnerable.

“The problem is that you can’t just pick this stuff up and ship it out of the country,” said one senior military official who has studied the problem. The chances of contamination of nearby Syrian towns, and of attacks on the effort to move the weapons, were simply too high. Because many of the containers holding the material are old, or of unknown reliability, the risk of leakage is high.

As a result, the official said, much of the chemical stockpiles might have to be destroyed in place. That is a lengthy, dangerous job, and would require enormous force protection around the sites. When the United States went through similar efforts to destroy its own stockpiles — under strict environmental regulations that would likely not apply in Syria — the process took years.

A second official familiar with the plans disputed the idea that all of the stockpiles would have to be destroyed in place. Some, he suggested, could be airlifted out for destruction elsewhere or burial at sea. “There are several options,” he said, “but all carry varying degrees of risk.”

That official said that rebel groups receiving nonlethal help from the United States have been asked to mark and secure any chemical weapons sites they come across.

The United States has varying estimates of how many sites exist, with the C.I.A. estimating about three dozen and the military using figures in the high 40s.

Officials said that the United States military had quietly sent a task force of more than 150 planners and other specialists to Jordan to help the armed forces there, among other things, to prepare for the possibility that Syria will lose control of its chemical weapons.

Elisabeth Bumiller and Michael R. Gordon contributed reporting.

A version of this article appears in print on November 16, 2012, on Page A4 of the New York edition with the headline: Pentagon Says 75,000 Troops Might Be Needed to Seize Syria Chemical Arms. Order Reprints|Today's Paper|Subscribe